Modern electronic equipment, and in particular handheld equipment, is often used in harsh environments in which the equipment is subjected to potential electrostatic discharge (ESD). For instance, data exchange ports such as those employed with universal serial bus (USB) or high-definition multimedia interface (HDMI) receiver/transceiver circuits are directly connected to external pins of electronic equipment. Current pulses from electrostatic discharge can have extremely fast rising slopes, such that protecting against such pulses requires rapid switching in order to shunt the current. In many instances, circuits are not robust enough to withstand the stress caused by ESD.
To address these problems, a variety of different types of ESD protection devices have been used, often implemented on a printed circuit board between external contacts and the integrated circuit of the device being protected. Such ESD protection devices generally shunt excessive currents to ground and clamp stress voltages to a level that the circuit to be protected can withstand. If the constraints on parasitic capacitance of the protection device are not stringent, simple p-n-junction diodes have been used.
If parasitic capacitance is desirably low (e.g., in order to not disturb high data rate signals), rail-to-rail or similar types of devices have been used. In such devices, two small steering diodes with small capacitance are often used for each channel, to shunt the stress current either to ground or to a large clamping device that shunts current further to ground while achieving a standoff voltage. Such clamping devices may include a simple diode or a more complex device, such as those in which a simple diode is used as a triggering component. The standoff voltage and the clamping voltage of the clamping device define the possible application.
The leakage current of the protection device at the standoff voltage (usually the supply voltage of the IC to be protected plus a safety offset) is desirably low where power consumption is a concern. Generally, the clamping voltage has to be kept lower than the acceptable voltage of the integrated circuit in which the device is used. Modern integrated circuits, however, have ever decreasing supply voltages and are more susceptible to high clamp voltages. Diode-based ESD devices often do not break down or otherwise operate satisfactorily at low operating voltages (e.g., below 6V). Other ESD devices can be difficult to manufacture in conjunction with standard integrated circuit processes.
Accordingly, achieving robust clamping while operating at low power has been challenging for a variety of circuits and ESD applications. These and other matters have presented challenges to ESD circuit protection, and related device operation.